Most of imaging apparatuses, including photographic film cameras and digital cameras that record electronic images in storage media, are provided with a focus control device for focusing an imaging lens on a subject automatically. There are various types of auto-focusing (AF) methods. Conventional digital cameras mostly adopt TTL contrast detection method, making use of an image sensor that is primarily used to take the electronic images.
In the AF process according to the TTL contrast detection method, image signals are obtained through the image sensor while moving a focus lens of the imaging lens step by step. Then, high frequency components of the image signals are integrated at each step of the focus lens. The integrated value is compared with other integrated values as obtained at different focus lens positions. Where the imaging lens is just focused on the subject, the contrast of the optical image of the subject gets the peak, and the integrated value becomes the highest. Thus, the in-focus position is detected with reference to the image contrast.
However, the contract detection method has a disadvantage that if the subject brightness is low the difference in contrast between the individual focus lens positions becomes too small to detect the in-focus position. To compensate for the disadvantage, some prior arts suggest projecting a supplemental light toward the low brightness subject, to raise the subject brightness and thus enhance the difference in contrast between the different focus lens positions, to enable detection of the in-focus position.
JPA 2002-341230 discloses an imaging apparatus, which makes a judgment as to whether the supplemental light is necessary for the AF process or not on the basis of a subject brightness value measured without the supplemental light. Thereafter when the supplemental light is projected, and if it raises the subject brightness too much to detect the in-focus position, the imaging apparatus of this prior art prohibits imaging. JPA 1996-160489 discloses an imaging apparatus, which projects a supplemental light to calculate a subject distance based on an incident position of the supplemental light on an image sensor as it is reflected from the subject toward the image sensor. Then a first flashlight is projected at an intensity decided by the subject distance. Based on the reflected amount of the first flashlight, the amount of a second flashlight is decided to make a proper exposure.
Since the in-focus position is detected as the position where the image contrast gets the peak while shifting the position of the focus lens step by step, it sometimes takes a certain time to detect the in-focus position. That is, the focusing time can be relatively long in the conventional TTL contrast detection method. In order to save the focusing time, an imaging apparatus as disclosed in JPA 2001-166200 divides the whole focusable subject distance range, i.e. from the shortest shooting distance to the infinity, into several distance zones so that the contrast detection AF process is executed within a chosen one of the divided distance zone. The choice of the distance zone is made on the basis of a difference between a subject brightness value measured without a supplemental light and a subject brightness value measured under the supplemental light.
In the above-mentioned method, as the contrast detection is carried out only in one of the divided distance zones, the focusing time can be saved if the subject actually exists in the chosen distance zone. However, if the subject does not exist in the chosen distance zone, the contrast detection is carried out again in another distance zone that is chosen in a sequence from more relevant zone to less relevant zone to the information on the measured subject brightness values. In that case, each time the distance zone for the contrast detection is switched over, the focus lens must be set to the initial position of the chosen distance zone. As a result, the focusing time is rather elongated.